NeXT slab SCSI fault

Anyone know if NeXT monochrome slabs have dual SCSI buses (one internal and one external) or not? I've got one here that resolutely refuses to talk to any internal SCSI device [1], but will happily talk to (and boot from, if necessary) external devices. In fact, if anything is plugged into the internal bus at all then the system won't even boot from an external device. I've ruled out internal cabling, internal power connector, something shorting the PCB underneath, and an actual device fault - which means it's possibly a bad solder joint somewhere or a cracked PCB track. However, *if* the slabs have two separate SCSI buses then it's possible that there's a fault with the actual controller too which is knocking out the internal bus. [1] Actually it "talks" in that there seems to be some device activity, but always falls over with a (useful!) "SCSI error" message whenever there's an internal device present. I'm not too fussed as the colour slab works and this one works too (albeit hanging the drive off the external connector), plus we've already got a good mono slab at the museum. It'd just be nice to get to the bottom of what's actually broken! cheers Jules